In many field operations there is a requirement for tracked and/or wheeled vehicles to traverse terrain which contains obstacles such as waterways, ditches or other similar topographical features which cannot be crossed by the said vehicles. The conventional method of negotiating such terrain is to span the said obstacles by means of a bridge structure which is brought up to the obstacle by a vehicle. The vehicle bringing the portable bridge must be provided with suitable apparatus to position the bridge to span the obstacle and the said apparatus must have the capability of recovering the bridge and transporting it to another site when required.
Known methods for accomplishing the bridging operation fall into two broad categories. FIG. 1 shows the first type where the portable bridge 1 is carried on the vehicle 2 (a tracked vehicle is shown by way of example) with the bridge in the same configuration it would take up when spanning the obstacle (i.e. “right way up”). In order to position the bridge over the obstacle, the load handling mechanism must translate the bridge horizontally and then lower it to the ground. To recover the bridge the reverse process takes place. Prior art of this type accomplishes the loading and unloading process by two translations, one horizontal and the other vertical.
FIG. 2 shows the second type where the portable bridge 1 is mounted on the vehicle 2 the “wrong way up”. The bridge has to be rotated through 180° and lowered down to ground level to span the obstacle. To recover the bridge the reverse of this process takes place. Prior art of this type accomplishes the loading and unloading process by a combination of a translation and a 180° rotation.
In prior art of the first type, translation is accomplished by a complicated rack-and-pinion mechanism driven by hydraulic motors, and the lowering by means of a system of levers. In prior art of the second type, both the rotation and translation processes are accomplished by a linkage system involving at least one pivotable structural element which is rotated by one or more hydraulic cylinders.
Bridges of this type have got longer as structural materials have improved. FIG. 3 shows a system capable of deploying a very long bridge. The bridge 1 must be located in a position such that when on top of the vehicle 2, the combined centre of gravity of the bridge and the deployment apparatus is over the centre of the wheelbase of the vehicle 2. This means that when the vehicle 2 is travelling without a bridge, the centre of gravity of the deployment apparatus is very far forward of that of the vehicle 2; this leads to unfavourable handling characteristics when the vehicle is in motion.
In FIG. 3, the vehicle 2 carries a bridge 1 in a stowed position. The bridge 1 is supported by the deploying apparatus which comprises of a ground engaging member (hereafter referred to as a foot) 3 a raising and lowering member 4, a bridge engaging member (hereafter referred to as a probe) 5 and three linear actuators 6, 7 and 8.
There are three stages to the deployment process:
First the bridge 1, the foot 3, the raising and lowering member 4, the probe 5 and the linear actuators 7 and 8 are rotated about axis 9 by the linear actuator 6 until the foot 3 presses on the ground as shown in FIG. 4.
Secondly the foot 3, the probe 5 and the linear actuator 8 are rotated about axis 10 until the foot 3 is flat on the ground (as shown in FIG. 5); this rotation being controlled by actuator 7.
Finally, the bridge 1 and the probe 5 are rotated about axis 11 until the bridge is in its deployed position (as shown in FIG. 6); this rotation being controlled by actuator 8.
The vehicle usually picks up the bridge after crossing it. The vehicle advances towards the bridge with the foot 3 just off the ground and the probe 5 lowered. Once the probe 5 has engaged with the socket on the bridge 1, it may be recovered using the reverse of the deployment process.
This prior art has the following disadvantages:                1) The three stage deployment process is complicated and heavy        2) It cannot be stowed on in a different position when it is not carrying a bridge—it remains a large structure projecting out in front of the vehicle, rendering it unfit for any other task.        3) It is connected to widely spaced pivots requiring that the roof of the vehicle be flat and low. Some of the pivots may be on the roof of the vehicle.        4) The linear actuator 8 has a very small moment arm when holding the bridge just off the roof of the vehicle, only about half of that when the bridge is fully deployed, while the moment of the bridge about the pivot 11 is the same in both cases. An excessively heavy, large bore cylinder is required, making the bridge recovery slow for a given hydraulic power.        
Preferred embodiments of the present invention seek to overcome all of the disadvantages of the prior art.